Spotlight Publication CGA S-1.1

CGA S-1.1 Guides Cylinder Pressure Relief Device Selection and Sizing

March 24, 2022

On February 23, 2022, the Compressed Gas Association (CGA) released the 16th edition of safety publication CGA S-1.1, Pressure Relief Device Standards-Part 1-Cylinders for Compressed Gases. S-1.1 is part of the S-1.X series published by CGA on pressure relief devices (PRDs):

A PRD is necessary to maintain pressure in a pressurized container below a level that could lead to its rupture – a catastrophic event that can harm people and damage equipment.

Each of the publications listed above focuses on determining the correct sizing of PRDs for a specific type of compressed gas storage container. CGA S-1.1 is the “go to” standard for manufacturers and users of PRDs on compressed gas cylinders, tubes, composite overwrapped pressure vessels (COPVs), and United Nations (UN) pressure receptacles.

“CGA S-1.1 is the ‘go to’ safety standard for manufacturers and users of PRDs on compressed gas cylinders, tubes, composite overwrapped pressure vessels, and UN pressure receptacles.”

CGA S-1.1 provides guidance for the selection of PRDs for more than 200 gases that are packaged in U.S. Department of Transportation (DOT) specification cylinders, in accordance with Title 49 of the U.S. Code of Federal Regulations (49 CFR) Parts 173.301, 173.304a, and 178.75.

In Canada, compliance with CGA S-1.1 is required (except for certain specified exemptions) in CSA B340, Selection and use of cylinders, spheres, tubes, and other containers for the transportation of dangerous goods, Class 2 and CSA B342, Selection and use of UN pressure receptacles, multiple-element gas containers, and other pressure receptacles for the transport of dangerous goods, Class 2.

CGA S-1.1 is referenced by the National Fire Protection Association (NFPA) and the International Code Council (ICC).

Overview of CGA S-1.1

This safety standard guides pressure relief device selection and sizing for cylinders that contain a single pure compressed gas. For guidance on PRDs for cylinders containing compressed gas mixtures, see CGA S-7, Standard Method for Selecting Pressure Relief Devices for Compressed Gas Mixtures in Cylinders.

CGA S-1.1 provides minimum requirements for PRDs used on cylinders with a water capacity less than or equal to 1000 lb (454 kg). This standard also applies to COPVs, DOT-3AX, DOT-3AAX, and DOT-3T cylinders with a water capacity greater than 1000 lb (454 kg), as well as UN pressure receptacles with a water capacity up to 3000 kg that comply with the design specifications, filling and maintenance regulations of U.S. DOT or Transport Canada (TC).

This standard defines test equipment and procedures that are applicable for PRDs with water capacities as noted above. CGA S-1.1 also provides sufficient test data to verify the capability of the cylinder system to prevent rupture of a normally charged cylinder.

For newly constructed cylinders that fall within the jurisdiction of DOT or TC, PRDs should comply with the requirements of this standard. The intent of CGA S-1.1 is to minimize the number, and optimize the types, of approved PRDs specified for each specific gas. This standard does not prohibit the continued use of previously approved and installed devices unless stated otherwise in CGA S-1.1, 49 CFR, CSA B340, and/or CSA B342. However, if a PRD is replaced, the new device should meet the requirements of this standard.

It is the filler’s responsibility to ensure that the correct PRD is used.

“It is the cylinder filler’s responsibility to ensure that the correct pressure relief device is used.”

How to Correctly Select & Size PRDs for Cylinders

CGA S-1.1 provides guidance on correctly sizing pressure relief devices for cylinders.

PRD sizing depends on the container’s contents and operation. To protect a cylinder, you must select a PRD with the proper set pressure and correct flow capacity. The PRD must open at the right setting to relieve overpressure. At the same time, the PRD must have enough flow capacity to relieve pressure faster than pressure can build up.

Both set pressure and flow capacity play critical roles in ensuring a cylinder’s safe and efficient operation.

Set Pressure

A PRD with a set pressure that’s too high – above the cylinder’s maximum allowable working pressure (MAWP) – will not protect the cylinder. The pressure can exceed the cylinder’s MAWP limit before the PRD can open and relieve the pressure. This over-pressurization can cause the cylinder to rupture.

Alternatively, a PRD with a set pressure that is too low – well below the MAWP – will not allow the cylinder to be used to its full capacity. While not a safety issue, operating a cylinder below its full capacity can lead to costly inefficiencies.

Flow Capacity

A PRD with a flow capacity lower than the maximum flow rate of an overpressure condition cannot maintain cylinder pressure below the MAWP. Pressure builds up faster than the PRD can relieve it. Eventually the cylinder pressure can rise high enough to cause a rupture.

A PRD with a flow capacity much higher than is needed will keep the cylinder pressure below the MAWP, so there is no safety issue. However, the larger PRD will cost more than the optimum-sized (smaller) PRD. This creates inefficiencies from having larger and more expensive components than needed, or could cause the PRD to “chatter,” opening and closing quickly, which can lead to premature failure of the PRD.

The Key to Optimum PRD Set Points and Flow Capacities

To find the optimum PRD set points and flow capacities to meet your specific needs, reference the CGA S-1.X standard that applies to your equipment. For guidance on pressure relief devices sizing for compressed gas cylinders, tubes, COPVs, and UN pressure receptacles, reference CGA S-1.1.

PRD set points and flow capacities depend on the following factors (this applies to all the publications in the CGA S-1.X series):

  • Type of vessel: gas cylinder, liquid container, gas receiver, tube trailer, liquid storage tank, liquid delivery vehicle, and others
  • Type of vessel design: liquid or gas service, vacuum-jacketed or not, type of insulation
  • Fluid stored in the vessel: flammable or inert, oxygen or not, liquid or gas
  • Types of upset conditions: fire, loss of insulation

What’s New in this Edition

Changes to the 2022 edition of CGA S-1.1 include:

  • Addition of composite overwrapped pressure vessel(s) to the definitions and scope
  • Modifications to pressure relief device discharge piping requirements
  • Revised requirements for tubes mounted in tube trailers or modules

For more details about this safety standard, the full Table of Contents may be downloaded for free from the CGA S-1.1 publication details page on the CGA portal.